Electroluminescent devices for use in electroluminescent displays, such as flat panel displays, are well known in the industry and are set forth in U.S. Pat. No. 5,247,190 to Friend et al.; U.S. Pat. No. 5,682,043 to Pei et al.; U.S. Pat. No. 5,723,873 to Yang; as well as Baigent et al. (1994), “Conjugated Polymer Light-Emitting Diodes on Silicon Substrates,” Appl. Phys. Letter, 65(21):2636-38. Such devices may be fabricated upon a suitable substrate in a multilayer thin-film configuration wherein a layer of electroluminescent material is positioned between an electron-injection and a hole-injection electrode layer in a “sandwich” configuration. When a voltage gradient is applied across the electrode layers, holes and electrons are injected into the electroluminescent material from the hole-injection and electron-injection electrode layers, respectively, which results in the emission of light through one of the electrode layers when the holes and electrons are combined in the electroluminescent material.
Commonly, electroluminescent devices for use in electroluminescent displays, such as those disclosed above, include an electrode that is transparent so as to allow light to be emitted there through. See, for example, U.S. Pat. No. 5,869,350 to Heeger et al. However, as very few transparent materials exhibit sufficient electrical conductivity to serve as such electrodes, the selection of materials that may be used is greatly limited. One such semi-optically transparent electrode material is indium tin oxide (ITO).
However, ITO suffers from several drawbacks with respect to its use as an electrode in an electroluminescent device. For instance, ITO is only a semitransparent material and therefore may act as a source of internal reflection when used as an electrode material in electroluminescent devices. Further, the conductivity of ITO is low compared to that of most metals. These characteristics result in a loss of transmission efficiency when ITO is used as an electrode layer. Additionally, ITO lacks chemical stability and thus its use as an electrode material results in a complicated fabrication process. Accordingly, ITO is not an optimal material for use in display applications.
Configurations other than the above described layered or “sandwich” structure have been proposed for use in electroluminescent devices. For example, U.S. Pat. No. 5,677,546 to Yu discloses a surface cell configuration that includes an anode, a cathode and an electroluminescent film, wherein the anode and the cathode are in electrical contact with the same side of the electroluminescent film. Further, Smela et al., (1998), “Planar microfabricated Polymer Light-Emitting Diodes,” Semicond. Sci. Technol., 13:433-39, discloses a diode having interdigitated electrodes that are capable of electroluminescence even though they are separated by a small distance. Further still, U.S. Pat. No. 6,593,687 to Pei et al. describes a cavity light emitting electroluminescent device (CLED) that includes a layer of dielectric material that is positioned between an electron-injection and a hole-injection electrode layer, and employs a cavity that is filled with a electroluminescent material. However, the configuration of one or more of these devices suffer from low brightness, as the actual light emission zone is only a small fraction of the device area, or require complicated fabrication methods that prohibit the use of low work function materials as electrode layers, e.g., as cathode layers.
Accordingly, there is a need in the art for an electroluminescent device that has a configuration which overcomes the inherent limitations of prior device configurations, utilizes a simplified fabrication process, and provides for a high-efficiency and bright display when employed in an electroluminescent display device. The present invention meets those and other such needs.